1. Field of the Invention
The present invention relates to furnaces. More specifically, the field of the invention is that of furnaces having control circuitry responsive to fault conditions which tend to stress the furnace heat exchangers.
2. Prior Art
Conventional furnaces include fans for circulating air about a heated plenum. For example, gas burners may be used to heat a heat exchanger which defines internal and external fluid passages. One fan, typically an inducer fan, circulates combustion air inside the heat exchanger for warming the body of the heat exchanger. Another fan, a circulator fan, causes the indoor air to circulate about the external surfaces of the heat exchanger so that the indoor air is warmed by contact with the heat exchanger body.
Conventional furnaces also include a thermostatic control having a temperature sensing element in thermal contact with the indoor air to determine if the indoor air requires heat. The furnace control activates the gas burners of the plenum and operates the fans according to signals from the temperature sensing element. A further feature of conventional furnace control circuitry involves additional temperature sensing elements which are disposed in thermal contact with the plenum to determine if the gas burners and the fans are operating properly. For example, a thermostatic control may include a high limit temperature sensing element or switch disposed in thermal contact with the plenum to determine if the plenum is too hot.
An example of such a conventional thermostatic control is disclosed in U.S. Pat. Nos. 4,976,459, 4,982,721, and 5,027,789, assigned to the assignee of the present invention, the disclosures of which are explicitly incorporated by reference. In the thermostatic control disclosed in the aforementioned U.S. Patents, a circulator motor fault signal is provided to the furnace control which indicates if the circulator fan is operative. In the case of an inoperative circulator fan which may be caused by motor or drive system failure, the control causes a flashing signal to appear on the control so that service personnel may easily determine the fault condition.
However, the failure of the circulator fan motor may have adverse consequences for the heat exchanger. Programming in the control circuitry halts normal operation of the furnace when the high limit switch indicates that the plenum is too hot. In the case of a circulator fan fault condition, the circulator fan is unable to circulate indoor air about the heat exchangers, causing the temperature of the plenum and particularly of the heat exchangers to increase. Eventually, the plenum temperature increases sufficiently to cause the high limit switch to open which then shuts down operation of the furnace. A potential problem with this conventional arrangement involves the rapid variations in temperature of the heat exchangers.
When the furnace first starts to heat, the heat exchangers initially have their temperatures increased rapidly and are thus quickly brought up to operating temperature. In the case of a circulator fan fault condition, the circulator fan does not cause air to flow over the heat exchangers, thus the heat exchangers are heated until the plenum is such a high temperature that the high limit switch is activated and the gas valve is turned off. The heat exchangers are then allowed to cool until the high limit switch is deactivated by heat dissipation within the building. The furnace goes through a ramping heat/cooling cycle repeatedly while the circulator fan is inoperative. This repetitive ramping heating/cooling cycle allows for some heated air to reach the interior of the building being heated, although much less than would reach the building interior with an operative circulator fan.
One problem with this sequence of operations involves the effect of the ramping heat/cooling cycle on the heat exchangers. Heat exchangers are conventionally designed to expand and contract because of the active and inactive periods of the furnace. However, when constantly expanding and contracting during a circulator fan fault condition, heat exchangers are subject to maximum thermal stress which fatigues the heat exchangers and shortens their useful life. For example, such heating and cooling may fatigue the sealing of clam-shell heat exchangers and prematurely separate the halves which define the interior combustion air passage. Another problem involves corrosion of the heat exchangers, which may be facilitated by additional condensate which periodically forms during the heat/cooling cycle.
What is needed is a thermostatic control which responds to a circulator fan fault condition.
A further need is a thermostatic control which minimizes the thermal stress on the heat exchangers during a circulator fan fault condition.